MK-0966 is a specific inhibitor of cyclooxygenase-2. It has recently been approved for use in adults. The pharmacokinetic (PK) profile of MK-0966 has not been assessed in children. The primary objectives of this study are to evaluate the steady state PK profile of MK-0966 in adolescent JRA children (12-17) compared to historical data from adults, to evaluate the safety and tolerability of MK-0966 in adolescents with JRA, and to gain experience in designing future studies with clinical measurements for assessing JRA. The hypotheses are that the PK profile of MK-0966 in adolescents with JRA will be similar to that observed with adult historical controls, when dose is adjusted for weight; and that administration of MK-0966 will be sufficiently safe and well tolerated by adolescents to proceed with further studies. This is a two-part protocol. Part I is a 14-day, open, oral dose, single-period study to evaluate the stead-state plasma-concentration profile of MK-0966 in JRA subjects aged 12-17. In Part II, subjects who successfully complete Part I will be eligible for randomization in a double-blind fashion to receive either MK-0966 at one of two dose levels or naproxen at one of two different dose levels for 12 weeks. We are applying to use the GCRC for some of the outpatient visits.